Sterilization with a germicidal agent, such as ethylene oxide gas or ethylene oxide containing gas mixtures, has played an increasingly important role in sterilizing heat or moisture sensitive materials. Rapid growth in the use of sterile, disposable medical devices is just one consequence of gaseous sterilization with agents such as ethylene oxide. Gaseous sterilization of re-usable medical and surgical equipment using a non-flammable mixture of ethylene oxide and a carrier gas has also proven to be reliable, cost effective technology for many hospitals.
The basic gaseous sterilization process consists of evacuating the sterilization chamber containing articles to be sterilized, preconditioning the articles at an optimal relative humidity, generally between 20-70%, admitting the sterilizing gas at an appropriate pressure and temperature, maintaining contact between the sterilizing atmosphere and the articles to be sterilized for an appropriate time, and finally discharging and evacuating the chamber to remove the sterilant gas.
Although there are many variations on the basic process, the major factors which have to be controlled in order to effect the sterilization are exposure time, temperature, ethylene oxide pressure or partial pressure, and relative humidity.
By itself, ethylene oxide is an extremely flammable gas. Its flammability range extends from about 3.0% by volume to 100% by volume in air. Thus, when ethylene oxide is used alone as a sterilizing gas, precautions such as explosion proof equipment are mandatory.
A preferable practice is to blend the ethylene oxide with another fluid which serves to dilute the ethylene oxide and render the mixture as a whole, nonflammable. Two such blends which have been used as sterilizing gases are dichlorodifluoromethane (CFC-12)/ethylene oxide and carbon dioxide/ethylene oxide. Inert carrier gases like CFC-12 and carbon dioxide inhibit the flammability of ethylene oxide and provide sufficient autogenous vapor pressure to deliver the liquid mixture from the source cylinder to the heat exchanger of the sterilizer vessel where the liquid mixture is vaporized.
A disadvantage of using CFC-12 in sterilant gas mixtures is that fully halogenated chlorofluorocarbons such as CFC-12 have substantial potential for stratospheric ozone depletion and global warming.
Although the major purpose of the inert carrier gas component in these sterilizing gas mixtures is to mask the flammability characteristics of ethylene oxide, simple substitution of an arbitrary nonflammable gas does not necessarily ensure a useful sterilizing gas mixture. First, the flammability properties of the blend must be such that a sufficient amount of ethylene oxide (mg/liter at a typical pressure and temperature) is delivered by the blend to effect the sterilization in an appropriate time. The Association for the Advancement of Medical Instrumentation (AAMI) recommends an absolute minimum ethylene oxide concentration of 450 mg/liter. If the carrier gas does not mask the flammability to a sufficient extent, a lower concentration of ethylene oxide must be used to ensure nonflammability. In such a case either a longer exposure time is required to perform the sterilization, which affects productivity, or greater operating pressures are required to increase the effective ethylene oxide density in the sterilization chamber. Increasing the operating pressure is generally not a viable alternative because existing sterilization chambers may not be rated for the increased pressure, and as pointed out by Gunther in U.S. Pat. No. 3,589,861, increased pressure can lead to swelling and rupture of the sealed plastic bags commonly used to package disposable medical devices. Indeed, lower operating pressures are advantageous in this respect.
A candidate inert diluent or carrier gas must also be miscible with ethylene oxide in the liquid phase and must not segregate from the ethylene oxide to any great extent during vaporization. Segregation or fractionation can lead to potentially flammable or explosive situations. The degree of segregation that may occur during evaporation is related to the relative volatility of the components of the mixture.
Thus, the need exists for an environmentally acceptable carrier gas which is compatible with the objects being sterilized; chemically stable; minimally segregating; contains at least 27 mole percent ethylene oxide; and provides sufficient vapor pressure to deliver the liquid mixture to the sterilization chamber.